Vertices that are not connect with bone get moved by bone
This is what I would do:

With the mesh selected, Tab into Edit mode. If the model snaps back to it's 'resting' state go into the modifier tab and on the Armature modifier click on the eye (Display modifier in viewport), the cube w/ 4 verts (Display modifier in edit mode) and the triangle (Adjust edit cage to modifier result), basically everything after the image of the camera.

You should now be able to see the exact vertices that are acting up. At this point you can select them individually and check the vertex groups that are affecting them.

Select a point and in the properties panel (n), near the top, under vertex weights you will see the offending bone in the list. Hit the 'X' next to it and watch the mesh snap back into place.

I hope this helps.